1. Field of the Invention
The present invention relates to a gas-tight pipe connection. The pipe connection includes a sleeve which has two internal thread portions which extend from the ends of the sleeve to a thread-free portion located in the middle of the sleeve. The sleeve further includes a radially inwardly projecting web located in the middle of the thread-free portion and extending over a certain width. The web has end faces constructed as contact shoulders. A sealing portion each is provided between the internal thread portions and the contact shoulders. The connection includes two pipes, each of which has in the end portion thereof a threaded portion constructed complementary to the internal thread portion of the sleeve. Each pipe also has a contact shoulder and a sealing surface arranged between the contact shoulder and the threaded portion.
2. Description of the Related Art
A pipe connection of a pipeline for use in the crude oil and natural gas industry composed of threaded pipes which can be screwed together has the purpose, on the one hand, to absorb the load in the line and, on the other hand, to secure the tightness of the connection. In order to meet this object, the threaded portions of the pipe ends and of the sleeve may have, for example, a conical API thread with an appropriate overlap. Because of the overlap of the thread, high surface pressures occur during screwing together of the pipes and the sleeve. In order to handle these high pressures, surface coatings and greases have been developed which make it possible that the components can be screwed together without jamming. These thread greases are spreadable pastes having a high proportion of finely distributed solid particles, such as graphite, metals, or teflon. In addition to ensuring a problem-free screwing together of the components, the greases also reinforce the hydraulic tightness in the areas of the threads of the connection, wherein the connection acts as a labyrinth-type seal.
In gas-tight pipe connections developed by various manufacturers, the tightness of the connection is usually obtained by a high contact pressure due to an overlap in the tight metal seat.
The contact shoulders serve as a stop means for the screw connection and ensure that the tight metal seat remains activated even when the load acting on the pipeline increases. Consequently, the conical thread does not have to perform a sealing function and, contrary to API round threads or buttress threads, only has the purpose to transmit the load in the pipeline. In the past, in order to reliably screw together the connections in the regions of the threads and the tight seat, surface coatings and lubricating pastes with solids were also used in these pipe connections in order to eliminate jamming and to ensure that the screw connections can be used again. The use of lubricating pastes with solids or screw greases in gas-tight connections with conical threaded portions has the substantial disadvantage that a high temporary sealing effect is created in the threaded portions which does not permit a test of the tightness of only the tight metal seat.
When a gas-tight connection is tested with respect to tightness after having been screwed together, the temporary sealing behavior of the threaded portions has the consequence that the test result does not provide a secure statement with respect to the tightness of the metal seal, even though the leakage test methods which have been utilized are capable of detecting leakage rates of less than 1 liter per year and the test pressures are in the range of the minimum internal pressure strength of the pipe or the pipe connection. Particularly in the case of low test pressures, the test period of 1 to 3 minutes which is available on the site is entirely insufficient for safely overcoming the temporary sealing effect generated by the screw grease. Therefore, as appropriate tests have shown, a gas tightness is very often only simulated when screw greases are used. These tests have further shown that the pseudo-tightness or false tightness increases with increasing pipe diameter because of the increased thread resistance and because of the usually additionally increased length of the thread. In addition, in larger pipes, i.e., pipes having an outer diameter of more than 7 inches and small wall thicknesses or small s/d ratios, wherein s is the wall thickness and d is the inner diameter, the test pressures used during the leakage test cannot be as high as compared to the pressures used in small pipes. Since the time during which the test gas passes along the length of the thread is essentially dependent on the pressure, it must be expected in larger pipes that the proportion of pseudo-tight connections increases.
One of the available possibilities for overcoming this problem is to significantly reduce the temporary sealing capability of the thread, for example, by using a low-viscosity oil, for example, slide track oil, as the lubricant instead of a high viscosity API grease, see German Patent 391 33 14. Contrary to the use of an API grease as the lubricant when the pipes are screwed together, the test gas used during the tightness test can penetrate within a short time through the oil film entrapped in the threads and can be detected if the tight metal seat should be leaking. However, this method can also not be used if the test pressure must be reduced to below 300 bar because the pipes have large diameters, thin walls, and a low material quality. Under these conditions, the time required for the test gas to flow through is increased even when a low viscosity oil is used to such an extent that the time exceeds conventional test periods. This means that, even when a slide track oil is used, it is possible that a pseudo-tightness is simulated, even if substantially longer test periods than are usual are employed. This would mean an additional unacceptable extension of the assembly times because of costs and the minimizing of danger.